Squash head projectiles had a soft explosive that on impact squished out into a platter shape before detonation, the shock wave travels through the armor and knocks a flake loose on the other side.
As I remember.....
You're exactly correct: known as HEP [High Explosive, Plastic] in US practice, HESH [High Explosive Squash-Head] to the Brits. The 76mm gun of the Saladin armored car was particularly effective with a HESH loading, developed in the days before spaced/Chobham armor was on the scene. Once it showed up, the Saladin was pretty much withdrawn from any frontline service.
But the energy from such an impact and explosion also transmits heat, as well as the friction of the *fleeks* of metal [love the term; gotta remember that one!] tearing themselves away from the interior surface combine to get the resulting sand-like chips hot enough to ignite fuel, hydraulic fluid. ammunition propellent or flesh. And remember that whatever pierced the Abrams was hot enough to set off the Halon fire extinguishers....
That could have been a HEAT round's Monroe effect jet, a self-forged beryllium/copper projectile from a Miznay-Schardin effect self-forged projectile...or maybe, spall from a HESH riund or a HVAPFSDS sabot penetrator.